1. Field of the Invention
This invention relates generally to voltage detectors and, more particularly, to an MOS circuit having an output stage which changes state when the circuit's own supply voltage crosses an internally generated reference voltage.
2. Description of the Prior Art
The advantages offered by MOS technology are well known; e.g. higher density, greater yield, etc. Thus, smaller MOS device geometries permit a greater number of devices to be produced per unit area, or, stated another way, a single MOS device will occupy less space. This characteristic is extremely important in the design and fabrication of complex digital integrated circuits; for example, single chip microprocessors.
Whereas digital circuitry is generally characterized by its "ON/OFF" or "ONE/ZERO" nature, most measurements in the real world are inherently analog; e.g. temperature, pressure, speed, voltage, etc. Therefore it is necessary that microprocessors and other digital circuitry communicate or interface with analog circuitry such as amplifiers, buffers, comparators, etc., in order to permit digital processing of the analog signals. The required interfacing may be accomplished by providing analog components which are external to the microprocessor chip. However, such arrangements generally require a larger power supply, more current, and commonly present more opportunities for design and manufacturing errors. To avoid these disadvantages, analog circuits are being manufactured integrally with the digital circuitry; e.g., on the microprocessor chip itself, and due to the complex nature of microprocessors, the inclusion of analog devices on the same chip requires that the same manufacturing process be employed.
As is well known, microprocessors are comprised of counters, registers and other digital circuitry, and it is necessary that such circuitry be initialized to a predetermined state whenever the supply voltage is below some minimum operational value as when power is first turned on and during power failures. Such circuitry has, in the past, been implemented with external discrete components such as timers and comparators in the power supply line which detect a low voltage condition. This, however greatly increases system costs, and users of microprocessor chips are reluctant to undergo the additional expense.